1. The Field of the Invention
The present invention is directed to the reproduction of temporary gray scale images on a poster sized display. More particularly, the present invention is directed to a poster printer that converts digital data into gray scale images using an electrostatic and magnetic toner deposited on an electrostatic background.
2. The Prior Art
Oral presentations to groups of people are often enhanced by including display graphics that the entire group can see. Numerous means for developing and presenting graphics have been developed. These include, e.g., the chalkboard, the marker board, the large tablet, overhead projectors for transparencies or opaque media, slide projectors, movie projectors, video displays for still and moving pictures (often using data transmitted to a remote location by satellite), special screens adapted to transfer data from a computer screen to the projection surface of an overhead projector, flexible marker boards that produce a photocopy of any writing on the board, poster display boards that produce an image from digital data by using electrostatic toner to form the image on an electrostatic sheet, and many others. The mere variety of image display systems suggests that each has limited applications and serious drawbacks.
The chalkboard is expensive, heavy, bulky, messy to use and difficult to clean.
The marker board, a metallic board, usually with a white writing surface, is quite expensive, requires the use of special and expensive markers (available in colors) that give off toxic fumes, is messy and presents the danger of permanently staining clothes.
The large tablet is sometimes effective but is often not large enough for a group of significant size, is bulky and difficult to transport.
Moreover, the effectiveness of all of these three media depends directly on the skill and clarity of the writing and drawing of the user. None of them is especially well-suited to producing different shades of colors. None of them can reproduce images derived from digital data, or any other previously prepared images.
Overhead projectors require a screen or screen-like surface for projecting useful images. The screen is typically permanently mounted on a wall or free-standing tripod. Overhead projectors are often used to project the direct writings of the user, and so their effectiveness often depends entirely on the skill of the user. Overhead projectors are also frequently used to project previously prepared permanent transparencies, to which the user can add his own comments and graphics. Such transparencies are expensive to produce and the blank transparency sheets themselves are expensive. Removing the marks left on them from one presentation in preparation for another is difficult and messy and is guaranteed to soil the user's hands and clothes. In addition, overhead projectors are bulky, noisy, and hot.
All of these display devices in ordinary use are limited to producing line drawings, i.e., solid lines, sometimes in color, on a contrasting background. In addition, none of these display devices can be utilized to create images from digital data readily.
Slide projectors are often used in polished presentations, but slide shows are notoriously expensive to produce notwithstanding modern computer techniques that reduce their cost. Typically, a very long lead time is required to produce them. A good projection screen is absolutely required for a good presentation. Movie projectors suffer from the same disadvantages, but producing serviceable movies is naturally much more difficult and expensive than producing slide shows. In addition, more skill is required to operate a movie projector than a slide projector. Movies are also difficult to store and transport, as is the projection equipment, and difficult to edit or change.
Video tape presentations have replaced movies in many applications. Producing video tapes is simpler and less expensive than producing movies, but video tape images are lower quality.
Slides, movies, and video tape are all generally limited to previously prepared material, usually prepared well in advance of the presentation and cannot be readily changed during a presentation. Moreover, none of these display tools can be conventionally used to produce images directly from digital data. Computer generated images can be transferred into these media, but only through an intermediary step.
Video presentations from remote locations are currently used but their use is seriously restricted by high cost. These are typically used to allow a remote audience to view a speaker. These systems operate on analog originals and so are not adept at producing images from digital data.
Special screens for transferring the digitally coded images onto the projecting surface of an overhead projector are available. They may either create the display directly from digital data from a computer by emulating a conventional computer monitor, or may merely transfer the visual image from the monitor itself. These systems, however, naturally require an overhead projector and so their use is subject to many of the limitations of the overhead projector. They can produce gray scale images but require projected light.
So called poster printers, such as those currently manufactured by Canon, Inc., produce an image from digital data by using electrostatic toner to form an image on a flexible sheet. Like all the systems described above except videos, slides, and movies (which are projected light images), they cannot produce gray scale images. This limits their utility in a visually oriented society that increasingly appreciates the value and information available from images produced over a wide range of colors or tones.
Efforts to reproduce images emulating a gray scale on the poster printer have proven only marginally successful. For example, halftones, which print black dots of various sizes to emulate a gray scale, have long been used in newspaper photographs and the like where necessity dictates that the ink dots cannot be allowed to run together. The halftone technique reduces the resolution of the image significantly.
Another technique, called dithering, uses varying patterns of black and white to represent gray, concentrating black dots in dark areas and reducing their concentration in the lighter areas. Dithering also seriously reduces the resolution and information content of the images. The loss of resolution is particularly troublesome in large displays. Both the halftone technique and the dithering technique produce images that appear very harsh and incomplete to most viewers particularly when viewed at close range, although the image may appear to be good when viewed from a distance.
Accordingly, there is a need for a large display medium capable of readily producing gray scale images from any digital data from any source of digital data, such as a computer or facsimile machine whether local or remote.